Multi-channel couplers have been conceived to meet the requirement which arises, when it is necessary to establish a conversational link via an optical channel between several terminals without passing through a switching centre, to be able to connect each of the transmitters to all of the receivers.
To avoid complex solutions which consist either in connecting each transmitter to each receiver through a separate optical channel or in dividing (by the use of 2.sup.n -1 cascade-connected splitter arrangements) the single optical channel leaving a receiver into 2.sup.n channels each terminating at a receiver, it has been proposed that a central mixer unit should be used which, on as many optical channels as there are transmitters, receives the signal sent by these latter, mixes them and then distributes them between as many optical channels as there are receivers. A unit of this kind is constituted by a light waveguide limited by a lateral cylindrical surface generally of right circular section, and by two flat terminal faces perpendicular to the axis, which constitute the input and output faces; the section of the waveguide is sufficiently large to arrange for it to be possible to dispose side by side on these faces the apertures of the fibres or bunches of fibres respectively emanating from the transmitters or going toward the receivers; in addition, the length of the waveguide is sufficient to contrive that the radiation emitted by a fibre, whatever its position on the entry face, covers the whole of the exit face, this consequently enabling the waveguide to perform its mixing function. It has also been proposed that a mirror should be arranged at one of the terminal faces; the other face then receives the transmitter and receiver fibres and consequently simultaneously fulfils the functions of input and output face.